The invention relates to a process for the solid-phase polycondensation of linear polyesters, especially of polyethylene terephthalate, which are continuously moved through a reaction vessel in granulated form in an inert gas stream at temperatures of 30.degree. to 5.degree. C below the melting point, sticking of the granulates being prevented by addition of fine-particle glass as an anticaking agent.
Polyethylene terephthalate can be condensed in the melt in autoclaves up to an intrinsic viscosity of maximally 0.7 and in reactors of special construction up to an intrinsic viscosity of 0.9, maximally 1.0, without suffering any substantial thermal damage. For certain purposes of use, however, a higher molecular weight is required, i.e., a higher intrinsic viscosity than is possible by condensation in the melt. In various technical areas there is an increasing need to replace inorganic materials with synthetic substances of especially high strength. By reason of a large number of favorable properties, including, for example, the lower density, there are many uses for high-molecular substances such as polyethylene terephthalate, polybutylene terephthalate, polyamides, etc. The strength of these resins depends on the molecular weight. An increase in the molecular weight over the limits set by the dynamic viscosity in the condensation in the melt can be achieved essentially only by a solid-state polymerization or polycondensation. For a number of uses polyethylene terephthalate having special properties is required, as for example, in the case of thin-walled packaging for foods. The requirements for this product will be discussed in detail below.
A number of continuous processes for solid-state polycondensation of polyethylene terephthalate are taught in the prior art.
German laid-open application OS No. 1,770,410 describes such a process, in which the granulate is moved by gravity in a plug flow. To be sure, as little as possible agglomeration is supposed to occur there, but it is proposed, through the admission of cold gas at the outlet of the reactor, to bring about a bursting apart of the particles, which nevertheless may be stuck together. The sticking obviously cannot be avoided completely. The process has the substantial disadvantage that the emergence of vapors such as the diol formed in the condensation is prevented at the agglutinated places because of the increase in the diffusion path length. As a consequence differing degrees of condensation occur in the various parts of the individual granulate particles, so that the end product has a very broad spectrum of viscosity or chain length.
During polycondensation an increase in density and a shrinkage in volume occurs and glacierlike cracks develop in the mass into which the granulate plunges from above. This results in the disadvantage that the otherwise plug-flow profile in the reactor space is disturbed, so that there is formed a polycondensation product of widely varied solution viscosity.
Discontinuous processes in which the granulate to be treated is prevented from agglutinating within a closed vessel by mechanical turning, for example, by agitating mechanisms, are not considered here, since other significant disadvantages arise, such as high power consumption and labor requirements, as well as a long reaction time. The trend, therefore, is toward carrying out the solid-state polycondensation continuously. Solid-state polycondensation processes which operate under high vacuum are excluded because of the problems of sealing the reaction space for continuous operation. Further, where leakage occurs there is a danger of the penetration of oxygen into the reaction chamber, whereby the polymer suffers damage by oxidation.